1. Field of the Invention
This invention relates to inkjet printing apparatus and methods for inkjet printing. More particularly, the present invention is directed to methods and apparatus for selecting the process parameters used in inkjet printing and for carrying out the selected process.
2. Description of the Related Art
Inkjet printing has increased in popularity in recent years due to its relatively high speed and excellent image resolution. Moreover, inkjet printing apparatus used in conjunction with a computer provides great flexibility in design and layout of the final image. The increased popularity of inkjet printing and the efficiencies in use have made inkjet printing an affordable alternative to previously known methods of printing.
In general, there are three types of inkjet printers in widespread use: the flat bed printer, the roll-to-roll printer and the drum printer. In the flat bed printer, the medium or substrate to receive the printed image rests on a horizontally extending flat table or bed. An inkjet print head is mounted on a movable carriage or other type of mechanism that enables the print head to be moved along two mutually perpendicular paths across the bed. The print head is connected to a computer that is programmed to energize certain nozzles of the print head as the print head traverses across the substrate, optionally using inks of different colors. The ink on the substrate is then cured as needed to provide the desired final image.
In roll-to-roll inkjet printers, the substrate to receive the printed image is commonly provided in the form of an elongated web or sheet and advances from a supply roll to a take-up roll. At a location between the supply roll and the take-up roll, a print head is mounted on a carriage that is movable to shift the print head across the substrate in a direction perpendicular to the direction of advancement of the substrate. Known roll-to-roll inkjet printers include vertical printers, wherein the substrate moves in an upwardly direction past the print head, as well as horizontal printers, wherein the substrate moves in a horizontal direction past the print head.
Drum inkjet printers typically include a cylindrical drum that is mounted for rotational movement about a horizontal axis. The substrate is placed over the periphery of the drum and an inkjet print head is operable to direct dots or drops of ink toward the substrate on the drum. In some instances, the print head is stationary and extends along substantially the entire length of the drum in a horizontal direction. In other instances, the length of the print head is somewhat shorter than the length of the drum and is mounted on a carriage for movement in a horizontal direction across the substrate.
Inks that are commonly used in inkjet printers include water-based inks, solvent-based inks and radiation-curable inks. Water-based inks are often used with porous substrates or substrates that have a special receptor coating to absorb the water. In order to cure the ink after it has been applied to the substrate by the print head, the printer often includes a heat source such as an internally heated bed or infrared heater in order to evaporate water in the ink.
Solvent-based inks used in inkjet printers are suitable for printing on non-porous films. Often, inkjet printers using solvent-based inks have a curing device that also includes a heater. The heater accelerates the rate of evaporation of the solvent and hence the curing of the ink after it is applied to the substrate. Such inkjet printers also often include an environmental system for collecting the solvent gases so that health risks to the operator are reduced.
Inkjet printers using radiation-curable inks have increased in popularity in recent years for printing on non-porous substrates. The use of radiation curing, such as ultraviolet (xe2x80x9cUVxe2x80x9d) radiation, enables the ink to cure relatively quickly without the need to dry large quantities of water or solvent. Commonly, the curing device of such printers comprises a radiation source such as a lamp that is spaced from the substrate a distance sufficient to provide prompt curing of the applied ink.
In modern inkjet printers, the speed of delivery of the printed image is considered to be of utmost importance. Still, care should be taken when selecting the combination of ink and the substrate, taking into account the type of printer to be used in printing the image. However, inkjet printers that are capable of printing on relatively large substrates are considered expensive. It is desirable to use the same printer to impart images to a wide variety of substrates using a wide variety of ink compositions if at all possible.
For example, the surface chemistry of the selected substrate can substantially affect the receptiveness of the substrate to the selected ink. This variation in receptiveness of the substrate to ink can vary not only from material to material, but also from lot to lot of the same class of materials. In addition, the behavior of ink on the substrate may be profoundly impacted by the selection of printing parameters of the inkjet printer that is used.
The printer operator is often provided with little guidance as to the selection of process parameters that will provide the best image quality for any combination of ink and substrate. Today, many operators use a manual trial-and-error methodology in an attempt to optimize the parameters of the printing process. For example, the operator may print a number of images and vary the curing time or temperature of the curing device. Once the images have cured, the operator visually reviews each image for image quality in order to help select an optimal temperature and/or curing time.
As can be appreciated, the manual methodology for selecting printing parameters as described above is somewhat time-consuming and tedious. This selection process is also subject to human error. In addition, the criteria for selection of the optimal image is somewhat subjective and may result in a substantial difference of image quality from one operator to another.
U.S. Pat. Nos. 5,508,826 and 6,039,426 describe automated methods for selection of printing parameters. In these patents, test pattern images are applied to a substrate and an optical detector examines the printed images. The detector is connected to a computer that, in certain instances, selects a printing parameter based in part on certain characteristics sensed by the optical detector.
However, there is a continuing need in the art to improve inkjet printers so that the final printed image is of satisfactory quality on a consistent basis. For example, the automated image quality detection techniques as described above do not necessarily assess all of the characteristics of the final printed image that may be needed to accurately determine whether or not the printed image will remain in a satisfactory condition over an extended period of time during its intended use. The longevity of the image quality is particularly important in instances where the image is to be displayed in exterior conditions, such as for use on an outdoor sign or banner used in advertising.
The present invention is directed to inkjet printing apparatus and methods for inkjet printing that include automated techniques for selecting and/or adjusting printing parameters. The resulting printed image is highly durable and exhibits a satisfactory image quality for extended periods of time. The present invention includes automated methods for altering test pattern images received on a substrate for assessing certain characteristics, such as adhesion of a particular ink to a certain substrate. A computer is used to determine which of the test pattern images exhibits optimal desired characteristics and then transmits signals that are used to adjust one or more of the parameters used in printing the final desired image.
In a particularly preferred embodiment of the invention, the printing apparatus includes a device for abrading at least a portion of the test pattern images. The device may include a stylus, a section of abrasive material or other structure that bears against the printed test pattern images with a certain pressure. An optical detector such as a spectrophotometer then scans the portion of the test pattern images that have been in contact with the abrading device, in order to ascertain which test pattern images have not been unduly damaged by the abrading device. The printing parameters used for such image or images are then used as a basis for selecting parameters for use in printing the final product.
In more detail, the present invention is directed in one aspect to inkjet printing apparatus that comprises a support for receiving a substrate and an inkjet print head for directing ink to the substrate received on the support in order to provide an image. A controller is connected to the print head for activating the print head to print at least two test pattern images on the substrate, and the controller causes at least two test pattern images to be printed using different printing parameters. The apparatus also includes a device for altering at least one of the test pattern images on the substrate, and a detector for detecting one or more characteristics of the test pattern images. The detector is connected to the controller and the controller is selectively operable to vary at least one of the printing parameters in order to achieve a desired final product.
Another embodiment of the present invention is also directed to an inkjet printing apparatus. In this embodiment, the apparatus includes a support for receiving a substrate and a print head for directing ink to a substrate received on the support in order to provide an image. The apparatus includes a device for contacting at least a portion of the image on the substrate, and a detector for detecting one or more characteristics of the contacted image.
An additional embodiment of the invention is directed toward an inkjet printing method. The method includes the act of selecting an ink, a substrate and a printer for printing the ink on the substrate. The method also includes the act of inkjet printing the ink in a plurality of test pattern images on the substrate with the printer, wherein at least one of the test pattern images is printed using one or more different printing parameters than the printing parameters used to print another test pattern image. The method also includes the acts of altering at least one of the test pattern images, and detecting one or more characteristics of at least one altered test pattern image. The method further includes the acts of correlating the detected characteristics of at least one altered test pattern image with one or more printing parameters, and selecting one or more certain printing parameters for the selected ink and the selected substrate based on the correlation. The method also includes the act of printing a final product with the printer using the selected printing parameters.
These and other aspects of the invention are described in more detail in the paragraphs that follow and are illustrated in the accompanying drawings.